1.Introduction
Power system protection is a specialised field in the area of Electrical engineering which deals with automatic tripping and isolating the faulted area of power system within the shortest time so that the fault does not effect the system and stability
The essential elements of protection is a sensor to identify the faulty condition and a device to initiate tripping signal to the Circuit breaker . The sensor is generally a current transformer(CT) or a voltage transformer(VT) or a combination of both .( There are other types of sensors like like bucholz ,oil temperature guage etc which initiates a tripping relay ) The signals from this sensors are given to the device called Relays which in turn process the signals and give the output signals based on the nature of fault to trip the associated circuit breaker in the system and isolate the faulty portion of the power system .
The initial relays were that of electromechanical and then to static relays and the numerical relays . Now the latest numerical relays are called Intelligent Electronic Devices (IED) which is , apart from capable of handling the relay functions are able to handle different control and automation functions . This latest IEDs are capable of communicating with each other and with the local PC and Remote control centre through various protocols including the latest IEC 61850. Hence now it is possible to set the relays , download the existing settings, down load a disturbance record , event logs ,read the measurements ,execute a control function by the operator sitting at the Remote control centre thereby completely avoiding the operator in the local substation.
The duties which were performed by the conventional SCADA system is thus completely replaced by the IEDs capable of performing the all the duties of the SCADA and many more .
As a protection Engineer you will have the responsibility from the application study , selection of relays , Selection of CT /VT , Sizing of the CT/VT ,designing the schemes ,preparing the setting calculation,conducting the coordination study and short circuit study, load flow study etc for the complete design of the protection system .
2 Application study
The application study and the selection of relays are interrelated - Application study is to be carried out to under stand a particular relay is suitable for the application in consideration . To identify this , the protection engineer has to understand the functions available in the relays , the setting ranges available etc . For example a transformer with three winding can not be protected by a differential relay having two inputs or with diff rent ratio available in the three windings , the range of correction factor available in the relay is suiting the requirement , the binary input and the out put are sufficient for the use,wheteher the relay output has heavy duty contacts which can be directly connected to the trip circuit with out going thru the trip relay or we have high set range available in the relay which meets the requirements of the system based on the result of a short circuit study etc.. Similar considerations are applied for each and every relays proposed to be used in the system
3. Selection of relays
The basic consideration in the selection of the relay is the application .In short the selection is based on the application study .Other words the details of the equipment to be protected ,how important the equipment to the system ,what is the economic aspect of the particular relay compared to the system stability and reliability point of view.
The philosophy to be followed for the protection of particular equipment is generally based on the utility practices and the local and international standards .IEEE has published various publication regarding the philosophy to be followed for various equipment and line protections .Also the recommendation of the relay manufacturer is a useful information in selecting a relay for a particular application.
4.Selection of Current Transformer ( CT)
The current transformers are the very important aspect of the power system protection . They are used to to step down the high current in the system so as to match with the relay or meter rated currents . The rated current of the relays are normally 1A or 5A and we have the CT with secondary values 1A or 5A primary rating depend upon the rated current of the equipment to be protected.
The CTs are broadly classified in to two types - one used for the metering purposes and the other for the protection purposes . The basic difference between the metering CT and the Protection CT is the saturation point ( to be discussed ). The metering CT goes to saturation soon after the normal working current so that the high value of fault current is not passed through the metering equipment which would otherwise have damaged the metering devices due to very high fault current . On the other hand the protection CTs are meant to reproduce the fault current and it shall not saturate ideally for the fault current ,and hence the protection CTs are having higher saturation point than the metering CTs.
5.Saturation of CT
The term saturation of the CT can be defined as a condition in which the CT is no longer able to reproduce the secondary current in proportion to its primary current . The basic principle on which the CT reproduces the secondary current is based on the well known Transformer principle where current flowing through the primary winding produces a flux in the core of the transformer which is normally proportional to the current in the primary. This flux is the basic reason for the current in the Secondary winding . Now if the core is not able to reproduce a flux in proportion to the primary current , then the secondary current will not be in pro potion to the primary current and hence the measurement of the current become wrong . The phenomena of the CT core unable to produce the flux in proportion to the primary current is called saturation of CT or CT core. This is a property of the core and depend upon the material of the core used in general.More interested readers are advised to go through the numerous literature available on this topic and is not discussed in detail here . However there are number of points to be understood for the reason for the current transformer to go to saturation which includes but not limited to the following
1. With constant VA applied in the CT secondary circuit the CT goes to saturation if the primary current is increased
2. With constant current in the primary and if the burden is increased , still we will find the same phenomena of CT saturation.
3. The amount of the DC component available in the primary fault current etc.
There are different methods to find out the saturation based on different standards like IEEE and IEC
As per the IEEE standard , it is the intersection of 45degree line with the magnetisation curve of the CT ( for non gapped Core ) and as per IEC it can be found out by obeserving the 50% increase in magnetising current for a 10% increase in excitation voltage.
Once the CT is saturated then it will not be able to reproduce the secondary current in proportion to its primary current and hence the relays connected to this CT circuit may maloperate indiscriminately and trips the circuit without coordination with other relays in the network or it may completely fail to operate for a fault both the condition to be avoided
Hence it is the duty of the protection design engineer to do a CT sizing calculation so that , the CT will not saturate for the worst case fault . There are number of factors to be considered before a CT is designed for protection purposes which included the maximum symmetrical and non symmetrical short circuit current and the system X/R ratio
The symmetrical and non symmetrical short circuit current can be calculated by doing a short circuit study on the system under consideration . For small systems manual calculation of the short circuit study is possible . However for large systems the application of software are generally required , otherwise the calculation become more tedious . Etap is one of the software using which we can perform the above studies()
6.Types of CTs for protection application
The Selection of CTs for the protection application is mainly based on the accuracy requirement , transient performance requirement etc . The simple over current relay which operates when the fault current or load current above the set value can be supplied with a CT with relatively less accuracy whereas the application like Distance relay which requires more accurate measurement of the fault current requires more accurate CTs.
7.Classification of CT for Protection
There are lots of types of CT classification based on different local and international standards . However the classification as given by IEC are of interesting and worth noting. According to IEC the classification is based on the steady state symmetrical primary current with out considering the transient performance and second case where the steady state performance and the transient performance is taken in to consideration.
Class P is an example where the error of the CT is defined with symmetrical primary current whereas Class TPX,TPY,TPZ etc are examples in which the transient performance is the criteria for the error.
even though the performance of the class P CT is based on the steady state symmetrical fault current , it does not mean that , this CT can not be sized for the transient application with suitable oversizing . However the transient performance is more clearly defined in TPX,TPY,TPZ with the limits of error , the remenent flux available , leakage flux in the CT etc as per the standard
There are number of applications in which we need to get a non saturated waveforms to the relay within first few cycles after the fault .As the maximum effect of the transients in the CT is during the first few cycles after the fault and it dies down with time.The effect of the transient in the CT is core saturation and hence ,the CT need to be sized adequately to overcome this effect
The basic reason for the transient in the CT is the time constant of the primary ( X/R)system up to the point of fault , the point of application of the fault on the the system voltage , the remenant flux available in the CT core , and the duty cycle of the CT etc
8.Protection schemes
There are different types of protection applied for different application in the power systems for example we have over head line protection, underground cable protection , Transformer protections , reactor protection , capacitor bank protection, Bus bar protection, generator protection and so on
Based on the equipment to be protected , there are number of protection methods and philosophies applied .Even though the philosophy of protection applied to a particular equipment is generally based on the utility practices , there are IEEE standards which discusses about the various protection philosophy that can be applied to a particular equipment . Before we proceed further on the philosophy of protection , it is better to have a understanding of basic protection principles of different type of protection .
9.Over current protection
As the name indicates , this protection is based on comparing the set current in the relay with the fault current and if the fault current is more than the set value in the relay , then relay will operate . There are basically two main settings to be applied in this type of protection ie , the current setting and the Time dial setting . If the relays are static or numerical relays , then we have the facility to select the characteristics in the relay like IEC standard inverse, very inverse , extremely inverse . definite time etc .
The time of operation of the relay is generally depend on the Time dial setting , current setting , and the characteristics selected or various combination of the above - For example if the definite time is the characteristics selected in the relay , then the relay will operate in definite pre set value of time if the current exceeds the set value of the relay . Or in other words the amount of fault current is not a criteria on the time of operation . As long as the fault current is more than the set value , the relay will operate in the preset value of time . On the other hand , if the charecteristics is selected as standard inverse , then the time of operation of the relay will be reduced as the fault current is increased .
The current settings are usually in Amps and is based on the relay setting calculation . Ideally it can be any current above the capacity of the equipment to be protected , however this setting is not straight forward .It depends on the coordination that must be achieved among the other Relays connected to the same equipment and the system as whole .
There are also directional over current relays which is designed to operate only for a particular direction of the current flow . To understand the direction of the current a reference quantity is required. Normally system voltage is taken as the reference quantity or we call by term polarising voltage .The idea of taking the system voltage as the reference quantity is based on the fact that , it does not change its phase angle appreciably during a fault . The direction of the current based on the selected voltage is found by the relay and the operation or non opertaion is decided based on this. Obviously such relays are supplied with CT and VT for its operation .
Cross polarising is the method normally used for the directional relays in which the reference voltage for the faulted phase is the other two phases . ie if there is a fault in phase A , the reference voltage used in this fault will be from the B and C phases. Its advantage to use such a cross polarising so that any change in the magnitude of the faulted phase voltage is not effecting the directional measurement of the faulted current
9.1Instantanious over current protection
Whenever the fault current is very high warrenting a fast operation to clear the fault , there is instantanious over current protection which is normally available as a part of over current relay . It is always a intresting point how to arrive at a proper value of instatntanious setting .The setting of this protection can be theoritically a value above the time delayed over current protection . However proper coordination between the downstream relays shall be done before selection a value for such a setting . For example if there is an over current relay in the primary of the power transformer , it shall be properly coordinated with a down stream setting in such a way that , for a through fault it shall not trip . To arrive a suitable setting for this protection , the power transformer through fault current shall be found out . As the instantanious protection is called upon to operate during the first few cycles of the fault current , the setting can be found out only by finding the both the symmetrical and the asymmetrical current and selected setting shall be above this value so that there is no false or non selective tripping of this protection with respect to the down stream protections.
10.High impedance REF protection
Before discussing the details of the REF protection it is better to have a good understanding between high impedance and low impedeance protection . It is a well known fact that the REF protection and the differential protections are basically working on the krichoffs current low ( there are volage differential relay also !)in which CTs are placed to measure the incoming current to the protected area and the out going current from the protected area . If this currents are equal then we conclude that there is no internal fault in side the area where the protection is supposed to be protected . obviously if there is any difference beteween the incoming and out going current then , theoritically we will conclude that there is a fault inside the protected zone
It is intersting to note that eventhough we are aware of the krichoffs current law applicable to a point or node , it is equally applicable to a surface. To understand the concept cleraly we can draw a complete closed path encircling the protected object and so that the drawn closed path appears like a surface . Now try to add all the incoming currents and outgoing currents algebrically ,then in all cases of normal system ( no fault ) this current adds to Zero ( of course it applies to only lenior networks and there shall be proper ratio factor applied if the surface includes a Transformer ).
Now coming back to the discussion on the REF protection, it is now clear that all the CT currents are added and they add up to zero under normal system condition . Now let us discuss about the causes of errors in this type of system under practical condition .
If all the CTs involved are of same characteristics then their error also become same and obviously there is no error current flowing through the relay and the relay will be stable . However if there is any mismatch among the charecteristics of the CTs involve inthe REF protection it will add to error current and and if this error current is above the relay setting the relay will maloperate under normal operating condition . The most important cause of error in a Ct is the magnetising current taken by the CT . ie the secondry current will not be propotional to the primary current devided by the transformation ratio but it will be lesser than this expecetd current by the magnetising current taken by the CT . Hence if the CTs involved having different magnetising current , then it will appear as a differential current in the relay and it will maloperate if the setting is below the maximum error current so obtained . Hence the setting of the relay is done based on the maximum expected error current in the CTs under consideration . In order to reduce the error current , the prefered CTs for this type of protection should have low magnetising current as possible.
Now it is understood that , if the CTs are not designed for the full expected through fault current , then there is every possibility for the relay to operate in case one of the CTs involved are operating in different operating point .
Now let us discuss about the high impedance resistor connected in series with the relay and its use in case of REF protection
During a through earth fault condition in which one of the CT is driven to saturation due to various factors like inadequate kneepoint voltage or a possible practical error in the CT material, construction etc , then only current from the othr CT is only present to the relay and hence the relay will trip as there is no current from the other CT to balance the healthy CT current . To overcoeme such a condition and to make the protection stable for such condition , it can be considered that the saturated CT is no longer working as a current source but it is acting as a simple winding with RCT ( resistance of CT ) . Hence the current from the other CT will also be flowing thru this resistance a making a voltage drop acros it . The maximum possible voltage across this resistance is found to be during a condition under which all the healthy phase CT current is flowing thru this saturated CT . The voltage across the saturated CT due to the current due to the healthy phase CT is found out and as this voltage is directly acting across the relay circuit , a stabilising resistance is added in series with the Relay circuit so that the current flowing thru the relay circuit is kept below the set current of the relay .
From the forgoing discussion , it can be concluded that there is error current in the REF relay during the following condition which requires special attention of the designer
1. The error current introduced due to difference in magentising charecteristics o the CTs involved - This can be over come by the basic setting of the relay
2. The error current introduced under fully saturated condition of one of the CT - This can be overcome by the proper designing of the stabilising resistor
3. The error current introduced due to partial CT saturation - this is a state of the CT in between the 1 and 2 above and appears to be critical . Under this condition the one of the CT error become very high so that , there is sufficient ciurrent which will trip the relay . The solution to the problems 1 and 2 will not automatically solve this critical issue but only by the proper design of the CT for suitable kneepoin voltage . This effect will be particularly severe during transient period ( switching or through faults ) Hence proper CT design for the trasient perfoermance is the only solution to overcome this problem . otherwise unwanted trip will result in the system.
10.1 A comparison of low impedance and high impedance protection based on the stability
In case of the high impedance REF or differential protection , the stabilising resistor is used to prevent the unwanted operation durring a through fault condition when one of the CT is completely saturated . Hence the circuit is stable even if the CT is completly sturated . However if the CT is not designed to meet the transient and steady state condition , there is a chance for a spill current if one of the CT is partially saturated . ie the CT is working in the upper non lenior portion of the magnestising current , Under this condition there can be considerable difference between the mag current of the CTs involved which may lead to tripping the relay if the error current is more than the set value
However in case of low impedance protection using the bias technique which is used to overcome the CT errors and mismatch of the CTS and not a measure for the CT full saturation . (If the bias charecteristics of the differntial relay is adjusted so that the tripping is prevented for one of the CT complete saturation, then it will not trip for an internal fault at all!)
10.2 Current setting of the REF relay
10.3Stabilising Voltage of REF relay
As discussed in the previous paragraph , when one of the CT in a REF circut is saturated , it will no longer work as CT and such a saturated CT can be replaced by its resistance . Hence the healthy CT current will be flowing through resistance of the saturated CT circuit . Hence as the relay circuit is in parallel with this CT circuit , the voltage that appears acros the saturated CT also appears acros the Relay circuit .This volatge is called stabilising Voltage . In order to prevent the operation of the Relay under this condition , the relay circuit burden is increased to a value so that the current in the Relay circuit is kept below the set current of the Relay . This is achieved by adding a resistance in series with the Relay circuit and this resistance is called stabilising resistance.
10.4 Sensitivity and magnetising current
The sensitivity of the REF protection depends on the magentising current of the connecetd CTs . The current that is available to the relay is the balance current after supplying the magnetising current of all the CTs. ie the current that is available in the secondary circuit of the CT is not equal to the step down current based on the transformation ratio of the CT but lesser than it be the magentising current required by the CT core . Hence if the fault current available in the primary of the CT is very low such that it is just sufficient for the CT magnetising curent ,there will be no current available in the secondray circuit for the relay to operate . Hence for such a low primary faullt current , the relay may not operate at all and in otherwise the relay is not sensitive to such currents .
Ideally for a internal fault and suppose the fault current supplied by nuetral CT and there is no current from the phase CTS . under such a fault condition the nuetral CT,apart from its own maganetising current has to supply the parallel conncetd CTs ie another 4CTs . Hence the relay will not operate for 4 times the magnetising current plus the set current of the relay ( converted to primary ). In otherwords the relay is not sensitive up to the above current.
10.5 Stabiliy and magnetising current
As in the case of sensitivity , the magnetising current also effects the stability of the REF protection . The CT sizing calculation is done for the through fault condition of the Transformer . ie the CT should be stable for a through fault condition . Under a through earth fault condition , the faulted phase CT has to supply the magnetising current of the unfaulted face as the the other two CTs will be clamped to the same pottential as that of the faulted phase . Hence this current ie the magnetisng current supplied to the healthy phases will be acting as the error current in the REF and if the relay setting shall be done above this current to avoid tripping.
10.6 Effect of value of stabilising resistance on stability and sensitivity
The value of stabilising resistor is foumd out based on the value of the stabilising voltage obatined when one of the CT is completly saturated . Now if the stabilising resistance is set at a higher and higher value the stability will be more and more for CT saturation . However for an internal fault as the burden seen by the CT is higher , there is a possibility for the CT to saturate and threby preventing the relay from operating
11.Transformer Differential relay
Transformer differential relay also working on the principle of current differential as in REF relay and they can also be high impedance and low impedance based . The high impedance principle is same as that of the REF principle as discussed above and the discussion in this topic is restricted to the low impedance principle
In order to make the relay stable for the differential current due to the slight mismatch in the primary and secondary current due to the effect of tap position during the normal operating condition and to account for the mismatch due to relatively large mismatch that would appear due to CT error and the slight saturation for through faults the relay uses the so called bias or retrain technique
The low impedance bias differential relay has basically 3 setting points and slopes . Before we proceed further on the matter , it is to be understood that , for every setting , the bottom line is that the Relay shall not trip under normal operation . Hence the limiting conditions are no load , load and the through fault of the transformer under which the relay shall not trip or in other words it shall refrain from operating . The more the settings are close to the above limiting condition , the more sensitive the relay operation is obtained . Hence we always have some fault condition under which the relay may not operate because the fault current is below the setting selected .
To understand the point more clearly , we can discuss the basic setting of the transformer differential relay . This setting is usually selected to avoid the tripping of the transformer under no load condition and to avoid any error introduced by the CTs involved . Generally 0.2 to 0.3 of the Full load current is sufficient to take care of the above . Now there is fault which is less than the above setting , then the relay may not operate for such low value of fault current .
11.1 Setting criteria
There are basically the following settings required to be done in a transformer differential protection
1. Basic setting
2. Slope of the first portion of the bias characteristics
3. The end of section of the above slope.
4. The slope of the second portion of the bias characteristic
5. The setting required for the unrestrained operation ( if available in the relay )
6. The setting required to block for the magnetic inrush
7. The vector compensation setting
8. The zero sequence current filtering setting
11.2 Basic setting
This setting should cover the primary magnetizing current of the transformer and the minor expected error of the Current transformer . Normally the magnetizing current is in the range of 5% of the full load current . Suppose a 40 MVA 66/11 KV transformer with full load current of 2100 A at the 66KV side then a 105 A of current is usually sufficient to set the basic setting . Suppose a CT ratio of 2500/1 A , then the relay need to be set at a minimum of 105/2500 = 0.042 A . But as a general practice it is set at 0.2 A to avoid any other error which would otherwise be introduced .
More accurately , the no load magnetizing current can be obtained from the FAT report of the manufacturer and the setting can be decided accordingly
11.3 Bias characteristics
Before discussing the the slope of the bias characteristics, it would be interesting to know basic principle behind the bias characteristics and its requirement in the transformer differential relay . The question here is what happens to the relay if it is set like a instantaneous over current relay which operates when the differential current crosses the set level .
Suppose one of the involved CT is getting saturated partially for a through fault and because of this saturation a differential current is measured by the Relay and if it is above the setting , the relay may trip unwontedly. To avoid such an unwanted tripping , a term called biasing is introduced in the relay so that the differential setting current is raised to a new value from the basic setting during such through faults . As the effective setting of the differential current is made a variable value by the application of the bias or restraining quantity in such a way that , the maloperation of the Relay is avoided during the through fault condition
Here the differential current which tend to operate the relay is determined by the phasor difference between the primary and secondary currents ( I1‾ +‾I2) and the restraining current which tends to hold the relay from operation is determined by averaging the primary and secondary currents algebraically ( Normally I1+I2/2) . Hence in the graph is drawn with differential current in the Y axis and the restraining current( bias current ) in the X axis . As the bias current I1+I2 /2 is nothing but a measure of the actual load current , the X axis of the graph can be treated as load current itself and the Y axis the differential current .
11.4 The slope setting of the first portion of the bias characteristics
The first portion of the slope characteristics is provided to get the stable operation during the actual loading of the transformer . During an actual loading , Normally the CT correction factor is applied in the primary CT and the Secondary CT when the tap is under normal operating tap . In other words it is done when the rated primary voltage is applied at the primary resulting in to the rated secondary current . But in actual condition there are chances for the primary voltage to change from the actual value . It may be lesser than the rated value requiring the tap position to raise from the normal or it may be higher requiring the tap position to lower . In both the cases if the load is assumed to be constant , the primary current is different from that of the normal tap and the secondary current is constant as there is no change in the secondary Voltage . Hence a relay which is designed to be balanced at the normal tap will find a spill current( differential current ) under tha above condition tend to operate . As the load increases , the difference also tries to increase . Hence the differential setting shall also increase to avoid the unwanted tripping . This is done by introducing a slope characteristics in the differential setting with respect to the load . The limiting criteria for this slope can be found out by various operating conditions under maximum and minimum tap position so that the tripping is avoided during the various operating condition of the Transformer . generally 30% of the setting will cover the stability in this region . If the slope of this setting is increased more than that is required , then the sensitivity of the relay to the fault condition reduces.
11.5 End of section of the first portion of the bias characteristics
As discussed in the previous section , the slope of the first characteristics determines the stability of the relay during the normal operating range of the Transformer . Hence end of section of the first slope characteristics is determined by the normal operating range of the Transformer with some safer margin. Generally 150 to 175 % of the normal load current is taken as the end of section of the first portion of the load characteristics. Any current beyond this point is generally taken as actual internal fault or through fault .
11.6 Slope of the second portion of the bias characteristics
One of the important setting requirement for the Transformer differential protection is to avoid the tripping during a through fault condition due to the CT saturation . Hence the second portion of the bias charecteristics determines the through fault condition . theoretically , the limiting condition for this slope setting can be found out by assuming that one of the CT is completely saturated .In this case the slope will be found out to be 200% ( that is the slope calculated with only one current is present and the other CT current is not available due to complete saturation of the CT ) But in practice if the slope is set based on the above criteria, then the relay will not operate for a internal fault fed from only one side . Hence the assumption made in this case is that severe saturation in the CT is not assumed but partial saturation so that the slope can be set a more realistic value for tripping on actual faults
11.7 The setting required for the unrestrained operation .
This setting is required for the relay to operate for without the effect of the restraining function for a heavy internal fault . The limiting condition for this setting shall be normally more than the through fault current of the transformer . If the relay see a current that is more than the through fault current , then it must naturally be an in zone fault current and the tripping shall be done more quickly with out the restraing force . Generally 120% of the through fault current is a good approximation for this setting.
11.8 The setting requirement to block the tripping due to magnetic inrush
The magnetic inrush current is the initial current flows to the transformer during the energizing of the transformer . This current will be normally very high and is available in the primary side ( or one side ) only and the relay will see it as differential current and trips the CB if proper care is taken to block the same . The magnitude of this inrush current depends on lot factors including the remenant flux available in the transformer core and the point of switching of the voltage . The important charecteristics of this current is that it contains second harmonic . Hence relays are provided facility to block the operation if the second harmonic component of the current is crossing a preset value . Now the modern power transformers are available with very low value as low as 5% of second harmonic component . Hence to avoid the tripping during the energisation , the second harmonic to the fundamental component shall be set accordingly
The FAT report of the particular transformer gives the available second harmonic component in the Transformer and the relay may be set accordingly
11.9 The vector compensation setting.
In a transformer there is always a phase angle difference between the primary and the secondary due to the connection of the winding .i.e. , the three phases of the primary winding can be connected in star and the three windings of the secondary can be connected in delta and similar combinations . Hence there is a phase angle difference between the primary and secondary windings current due to the above . As the relay is set for the differential current which is set to operate for the magnitude and the phase difference between Primary and secondary currents , it will mal operate and trip the breaker even if the primary and secondary currents equal in magnitude if the phases angle of the primary and secondary currents are not made same . Interposing CTs are used to introduce this vector compensation in Electromechanical Relays . However with the arrival of various numerical relays , there are software interposing CTs available inside the Relays which can be set to compensate for the vector difference between the primary and secondary based on the actual phase difference .
11.10.Zero sequence filtering
The basic requirement of the stability of the relay during the through fault is achieved based on the applied CT correction factors in the relays ( of course with proper vector compensation). By applying the proper correction factors the Current from the primary and secondary are made equal to rated current of the relay under full load condition .In other words , the primary current is the reflection of the secondary current and the secondary current is transferred to primary based on the transformation ratio of the Transformer . By selecting proper CT ratio and CT ratio correction factor the currents in the primary and secondary are made equal and applied to the Relay Hence they cancel each other at the differential element and the relay is made stable . However this stability is valid only for the three phase loading or three phase fault condition. If there is a single phase loading or single phase to earth fault in the star side of a delta – star transformer with star earthed , then we can see that , this earth fault current is not reflected to the primary based on the actual transformation ratio of the Transformer . Obviously the relay will not be stable . Hence we go for a method to make this current equal under this condition also and is called zero sequence filtering.
The word zero sequence filtering is based on the fact that , the earth fault current is zero sequence current and hence if proper methods are taken to equate the primary and secondary current fed to the relay called zero sequence filatering
Traditionally , the zero sequence filtering and vector compensation and amplitude matching is achieved by using a Auxilary CT ( matching CT )conncetd star star in delta side of the main transdormer and star delta Aux CT in Star side of the transformer with Auxilary CT delta side connected to the relay
It is important to note that , there is no phase shift between primary and secondary current during an earth fault ( except the 180 degree normal shift )in the star side of the power transformer with star earthed . ( even though there is a phase shift when there is 3 phase current present ).
In numerical relays the matching factor , the vector compensation and the zero sequence filtering is done by software methods . The method of zero sequence filteration and vector compensation differs based on the phase shift between primary and secondary.
12. Distance Relays
As the name indicates , the distance relay work on the principle of measuring the distance from the relay location to the fault. In otherwords if we know the votage across the fault and the current flowing through the fault , the ratio of this voltage and the current gives the impedance of the line and hence the relay can take a decision on tripping the associated CB. With obvious reason the relay requires the Voltage input from the VT and the current input from the CT to process this information to arrive at the fault .Unlike distance relays which can be set to operate above the load current , the distance relays can operate for a current less than the load current
Hence in a distance relay the settings are already given based on the line impedance and this value is compared with the measured value using the VT and CT input and if the measured impedance is less than the set impedance , the relay take a decision to trip the associated Circuit breaker
To understand the principle of Distance relays let us Assume a radial feeder , with a infeed from one side with VT and CT connected at the feeding side . Now let us assume there is a line to line fault at some point on the line , now the current seen by the CT is the fault current and the Voltage across the VT is the drop across the line up to the fault and that of the return path ( loop ) . Now if we calculate the ratio of voltage and current , then we get the impedance of the loop of the line and dividing this result with 2 , we get the impedance of the line up to the fault .
Similarly , if there is an phase to earth fault , then the voltage seen by the VT is the voltage drop in the line up to the point of fault and the drop of the earth return path . As the impedance of the earth return path is not equal to that of the line , the above mentioned method of measuring the impedance by direct method will not be applicable here and to get the actual impedance up to the fault , a factor called residual compensation factor to be given to the relay to calculate the actual impedance up to the point of fault . It is to be noted that , irrespective of nature of the fault whether it is line to line or line to earth , the point of fault is measuredby caluculating the impedance of the line up to the point of fault . In short if the impedance of the line up to the point of fault is Z1 and for a line to line fault at this point or line to earth fault at this point should give the same result for the fault location ( That is Z1 in case of line to line fault and the same Z1 in case of line to earth fault ) .
Residual compensation factor .
As explained in the forgoing paragraph , for a line to line fault , the impedance up to the point of fault can be calculated more directly where as for a line to earth fault , the impedance up to the point of fault can not be found out directly due to the different impedance of the line and that of the earth return path . Let us analysis the above by a single line to earth fault with line impedance of Z1 and earth return path impedance of ZE
Now for a single phase to earth fault we know that the loop impedance is Z1+ZE and the voltage drop across the faulted loop is
Vph.n = IaZ1 + InZE
= Z1( Ia + In ZE/Z1)
Where Ia is the line current and the In is neutral current or Earth fault current ( of course in this case Ia=In and In is used in the equation for more generalization of the result )
Now from the above equation ,
Vphn/Ia+ In ZE/Z1 = Z1 which is the impedance of the line up to the point of fault
We know that Vphn is the line to neutral Voltage of the VT and the Ia is the current measured by the CT . It is clear from the above equation that , in thedenominator of the equation there is second term apart from the CT current Ia , that is a component of neutral current or residual current . The factor by which this neutral current is multiplied (ZE/Z1) is called residual compensation factor
Now for a transmission line or cable we know that , the zero sequence impedance Z0= Z1 +3ZE ( by definition )
Hence ZE = Z0-Z1/3
Substituting for ZE ,then we have another equation for residual compensation factor as Z0-Z1/3Z1
Zero sequence compensation factor
From the foregoing discussion , the poertion of the neutral current added to the fault current Ia to obtain Z1
= In ZE/Z1
= In Z0-Z1/3Z1
= In/3 Z0-Z1/Z1
= I0 Z0-Z1/Z1 where I0 is the Zero sequence current which is equal to In/3
= I0 K0 ( where K0 = Z0-Z1/Z1 is called Zero sequence compenstation factor )
Zones of protection
The protection Zones in case of a distance relay applied to a transmission line is generally devided in to 3 zones
Zone 1 - The measured impedance of the relay depend upon the measured value of voltage and the current . Naturally if there is any error in the measurements these error will also be introduced in the measured impedance . Hence a relay which is supposed to measurre the fault impedance may see an impedance more than that of the actual impedance or it may see an impedance lees than the actual impedance . In otherwise the relay may under reach or it may over reach . Hence to take care of the situation and to avoid the relay overreach ( to avoid the relay to see the fault in the next line from the substation ) an allowance is usually made in the setting . Hence if it is intend to protect a line in between two stations , the setting is usually done to cover 70-80 percent of the line .This is called Zone 1 seting .The timing is usually instatanious for this zone . The remaining portion of the Zone is covered by a time delayed Zone 2 which is set to cover 100% of the protected line plus 50% of the next shortest line so that a fault between 80 % to 100% of the line is seen by the time delayed Zone 2 The normal timing applied for the zone 2 protection is 400msec .There is another backup zone set in the relay known as Zone 3 which is set to cover the 100% of the protected line and 100% of the next largest line from the remote station . This timing is usually set to 800msec . Care also should be taken to avoid the zone 3 setting to cover the fault impedance . Hence the maximum loading condition shall also be taken in to consideration before setting the Zone 3 reach ( otherwise load will be treated as fault and results in unwanted relay operation )
Zone 4 settings
There is an another setting above the Zone 3 but below the load impedance which is used to detect the power swings in the system which characterized by the slowly varying impedance which passes through different Zones . During a fault the fault , the change from the load impedance to the fault impedance is very fast whereas in case of power swings the variation of impedance during the power swing is in comparatively slow . Hence the power swing is detected by the time taken for the impedance curve to cross from the Zone 4 to Zone 3 and decision is taken by the relay accordingly.
To be continued....
